Turbine type flow meters have been used for many years in the measurement of fluids and this type of metering has become increasingly popular because of its simplicity, repeatability, reliability and the relatively greater accuracy which turbine meters provide over other forms of meters particularly at large quantities of flow.
It is generally understood in the art that each meter which is manufactured and assembled in accordance with conventional methods has its own unique registration or calibration curve. At the time of manufacture the actual flow through the meter is determined by a flow prover placed in a series in the test line with the meter being calibrated. A flow prover is a highly accurate instrument which itself has been calibrated to measure to a high degree of accuracy the quantity of flow. Meters produced by conventional manufacturing methods will each show a slightly different quantity of flow for the same quantity as shown by the flow prover. This is caused by a number of factors. For example, the different sets of bearings in one meter may impose a slightly different drag on the rotation of its rotor than the bearings in other meters will impose on the rotors with which they are associated. Also the angles at which the blades are oriented with respect to the direction of fluid flow may vary slightly from meter to meter as will the area of annular flow passage through which the fluid flows as it passes through the meter. As a practical matter, it is impossible under conventional production methods to maintain the effect of these factors precisely the same from meter to meter. Also, the mechanical load imposed on the meter by the various drive elements such as gears, magnetic coupling, and so forth between the rotor itself and the registering mechanism will also vary from meter to meter. Thus, variations in these factors from meter to meter result in each meter having a unique value of flow through the meter for a given quantity of flow as measured by the prover. The ratio of the meter reading at any given rate of flow to the prover reading is referred to as the "percentage of registration." Thus, a meter which shows a registration of flow of 999 cu. ft. of flow when the prover shows a flow quantity of 1,000 cu. ft. is said to have a registration of 99.9%; that is, it registers 99.9% of the fluid which actually flowed through the meter. The curve produced by plotting the percentage of registration of a meter at various rates of flow throughout its stated range of operation in terms of flow rates is called the calibration curve and each meter has essentially its own unique calibration curve.
In the field, therefore, if after a given period of time the meter shows on its indicator a quantity of 10,000 cu. ft. of fluid having flowed through the meter at a given flow rate and if at that flow rate the percentage of registration is 99.9%, the actual flow through the meter is 10,000 divided by 0.999 or 10,010 cu. ft. of fluid. As stated above since the calibration curve shows the percent of registration for the various flow rates throughout the operating range of the meter, by dividing the value shown on the meter register by the percentage of registration as shown on the calibration curve, for that meter at the flow rate the system was operating, the actual flow through the meter can be calculated.
In the course of the extended field use of the meter, any one or more of the factors mentioned above which influence the calibration curve can change. For example, the rotor bearings may wear due to their continuous use, resulting in much larger bearing friction than when they were new, foreign material in the fluid being metered can become lodged in the bearings, or the annual flow area may change because of the accumulation of foreign matter, causing a change in the influence which those particular factors have on the amount the meter shows on its register for given amount actually passed through the meter. For example, if the bearing friction has increased due to continuous use to impart a considerably greater load on the rotor, then instead of registering 99.9% registration in the example given above, the register on the meter may show only 98.9% of the fluid actually passed through the meter. In such a case the meter would register 1.1% less than 10,000 or 9.890 cu. ft. Since the operators have no indication that the meter is not operating in accordance with its calibration curve, the reading of 9,890 would be divided by the normal percentage of registration figure of 99.9% which would give a spurious result of (9890/0.999)=9900 cu. ft.
In the past it has been the practice to periodically remove the meter from the line and to recheck it and recalibrate it against the standard of a meter prover. This, of course, requires considerable time and expense and often results in the meter being operated while out of calibration for extended periods of time between calibration checks. In U.S. Pat. No. 4,091,653 assigned to the assignee of the present invention, a method and apparatus is disclosed for checking the accuracy and calibration of a turbine meter without removing the meter from the line and without the need of interrupting its normal service. As described in that patent, it has been found that changes in the calibration or the percentage of registration of the meter result in changes in the angle at which the fluid exits from the blades of the metering rotor. Thus, if at the time of original calibration the exit angle of the fluid leaving the rotor is noted and specified, by periodically checking the exit angle of the fluid while the meter is in service any deviations in the exit angle of the fluid from that specified at the time of original calibration will indicate to the operator that the meter calibration has changed. That patent disclosed means provided within the meter to provide an indication of the exit angle of the fluid. The instant invention is an improvement to the invention disclosed in that patent and provides a means of continuously monitoring the exit angle of the fluid so that when changes in the exit angle are sensed these changes are used to correct the registered quantity of fluid in accordance with such changes to provide a continuous and accurate registration of the flow thru the meter.
Prior attempts to achieve high accuracy in turbine meters are shown in the U.S. Pat. No. to Souriau 3,142,179 and the U.S. Pat. No. to Griffo 3,934,473. The patent to Souriau discloses a turbine meter in which the fluid entering the meter is given a tangential velocity by means of fixed angularly oriented vanes. The fluid which then has a tangential velocity component impinges on the vanes of a metering rotor causing it to rotate. According to the teachings of that patent the meter operates at greatly enhanced accuracy when the tangential velocity component is completely removed by the metering rotor. A brake is provided which is adapted to apply a braking torque to the metering rotor the magnitude of the torque being adjustable by rotation of a sensing rotor which is provided downstream of the metering rotor. If the fluid leaving the blades of the metering rotor has any tangential velocity component left which has not been removed by the metering rotor, the sensing rotor will be caused to rotate. Rotation of the sensing rotor varies the amount of braking effort which is applied to the metering rotor until the metering rotor is rotating at a speed at which all of the tangential velocity component is removed from the fluid exiting from the blades of the metering rotor. In the present invention no tangential component is imparted to the fluid entering the metering rotor vanes and no attempt is made to remove the tangential component of velocity of the fluid leaving the metering rotor blades.
The patent to Griffo discloses a turbine meter in which a sensing rotor downstream from the metering rotor is adapted to rotate in a direction opposite from the direction of rotation of the metering rotor at approximately the same speed as the metering rotor, the speed of the sensing rotor varying with changes in the speed of the metering rotor. In accordance with the invention disclosed herein it is shown to be advantageous for the sensing rotor to operate in the same direction as the metering rotor at a considerably reduced speed.
Other patents typical of efforts to enhance the accuracy of turbine meters are the U.S. Pat. Nos. to Allen 3,241,366 and Hammond et al. 3,710,622.